In a typical cellular network, also referred to as a wireless communication network or system, User equipment, UEs, communicate via a Radio Access Network, RAN, to one or more core networks, CNs.
A user equipment is a mobile terminal by which a subscriber may access services offered by an operator's core network and services outside operator's network to which the operator's RAN and CN provide access. The user equipment may be for example communication devices such as mobile telephones, cellular telephones, smart phones, tablet computers or laptops with wireless capability. The user equipment may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server. The user equipments are enabled to communicate wirelessly in the cellular network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the RAN and possibly one or more CNs, comprised within the cellular network.
The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a Radio Base Station, RBS, which in some RANs is also called eNodeB, eNB, NodeB, B node or network node. A cell area is a geographical area where radio coverage is provided by the radio base station at a base station site. Each cell area is identified by an identity within the local radio area, which is broadcast in the cell area. The base stations communicate over the air interface operating on radio frequencies with the user equipment within range of the base stations. It should be noted that a base station may serve one or more cells, also referred to as cell carriers, within its cell area.
Multi-User Multiple Input Multiple Output, MU-MIMO, is technology that may be implemented in a wireless telecommunications network. This allows distinct UEs to transmit and receive separate data streams to and from a central Access Point, AP, such as, e.g. an eNB in a Long Term Evolution, LTE, wireless communications network. When used for uplink transmissions, MU-MIMO may sometimes also be referred to as virtual MIMO. This is because multiple UEs, which potentially may only be equipped with a single transmit antenna, may effectively form a MIMO system similar to a single UE that is equipped with multiple transmit antennas. The difference is that, in MU-MIMO, the different transmit antennas may belong to distinct UEs.
MU-MIMO is attractive because it may take advantage of spatial separation of multiple UEs and multiple data streams. Thereby, it may also increase the spectral efficiency in the wireless communications network. Hence, MU-MIMO may also facilitate the reuse of radio resources at the expense of a minor increase of the intra-cell interference.
It follows that in a wireless communications network using MU-MIMO, an important issue is how to select the group of UEs that is to share radio resources at a given point in time. This UE grouping or MU-MIMO scheduling is often controlled by the Medium Access Control, MAC, layer in the network node. It is typically also not incorporated into any standard specifications, but nonetheless form an important proprietary part of MU-MIMO implementation in a wireless communications network.
Device-to-Device, D2D, communications of user equipments is also a technology that may be implemented in a wireless telecommunications network. This allows user equipments that are in the proximity of each other to discover one another, and establish a direct link to each other rather than a link via a network node. The setup of this direct link of the D2D communication may be assisted by the wireless communications network.
The direct link of a D2D communication between user equipments may also reuse the same radio resources that are used for cellular communications in the wireless communications network, either in the downlink (DL), uplink (UL) or both. Typically, D2D UEs use UL radio resources of the wireless communications network in their D2D communication, such as, e.g. UL PRBs or UL time slots, but may possibly also use DL radio resources, such as, e.g. the cellular DL spectrum or DL time slots.
This reuse of the same resource may lead to an increase of spectral efficiency in the wireless communications network, but also here at the expense of an increase of the intra-cell interference. From a resource reuse point of view, there is thus some similarity to the resource reuse in MU-MIMO, with the difference that in D2D communications the communicating UEs are distinct, whereas in MU-MIMO the separate UEs may all communicate with the same physical entity, such as, e.g. an LTE eNB.
In a wireless communications network supporting both MU-MIMO and D2D communications in its cellular spectrum, it is thus even more important to address the issue of how a network node should allocate radio resources, transmit powers, etc., to a set of UEs comprising both MU-MIMO and D2D users. This may also be described as how the network node should schedule a group of MU-MIMO and D2D UEs that share the same or overlapping resources for signal transmission in the wireless communications network. If not solved properly, this may lead to severe increases in interference, degradations in spectral efficiency and capacity, and/or problems with Quality-of-Service, QoS, at the UEs. It is also clear that the conventional multiplexing in MU-MIMO systems, i.e. using diversity—spatial multiplexing trade off, is going to be affected by the presence of non-orthogonal D2D communications between UEs in the cellular spectrum.